Conventional helical filter includes two or more helical coils, each coil being housed within an enclosed resonating chamber. The coils are positioned relative to each other spatially and electrically coupled through apertures in the walls separating chambers to provide characteristic bandwidth. The maximum bandwidth of the filter so formed is determined by the geometry of the coils and the chambers and the size of the apertures. The required coupling of the elements in a helical filter is achieved by adjusting the size of the apertures between adjacent resonators.
The increase of the exposed parts of one helix to the adjacent helix causes increase in coupling resulting in larger filter bandwidths. In the extreme case where the walls between the filter elements are completely removed, the maximum coupling for the given size of enclosure and coils is achieved. Additional increase of filter bandwidth can be obtained either by increase of the size of the filter elements, which is not always possible, or by some means of further increase in coupling. Thus, given a physical size of the filter, the maximum bandwidth that can be afforded by the conventional helical filter is fixed.